Adequate P and K levels critical for high-yielding alfalfa stands

WEST LAFAYETTE, Ind. — Alfalfa growers need to start thinking about whether they should apply potassium and phosphorus on their fields this spring and at what rate, said two Purdue University experts.

Both phosphorus and potassium can increase yield and stand longevity, but to maximize production and profitability it’s important to adjust fertilizer rates to meet the nutritional needs of plants in each field.

Balance is critical

Recent research at the Throckmorton-Purdue Agricultural Center showed a balance of the two macronutrients is critical to keeping a healthy alfalfa stand.

“Alfalfa removes more potassium than any other field crop, with the exception of corn silage, so growers need to pay close attention to potassium,” said Jeff Volenec, Purdue Extension crop physiologist.

“An imbalance between potassium and phosphorus fertilizer can actually be more damaging when you apply phosphorus and do not apply potassium — the stand thins out more quickly, which results in added weed encroachment.”

Soil test

A soil test should have been done last summer or early fall to determine the appropriate application rate, said Keith Johnson, Purdue Extension forage expert.

Soil tests done in the spring do not accurately reflect the amount of potassium available to the plant, he said.

However, if a soil test was not done last year, growers should take a sample after the first harvest, Johnson said.

“It’s important to have a soil test because serious consequences, such as milk fever in dairy cows and grass tetany in beef and dairy herds, can occur from applying too much potassium,” Johnson said.

Application

The application of potassium and phosphorus should be split — one half the total annual amount applied after the first harvest and the other half after the last harvest — to avoid luxury consumption, Volenec said.

Luxury consumption means the alfalfa plant will take up more potassium than it needs.

Anything more than two percent potassium in the forage on a dry weight basis is a waste of money and potentially dangerous to the livestock grazing on it, Volenec said.

For highly productive fields producers can anticipate applying a total of 250 pounds of potassium per acre and 25 pounds of phosphorus per acre, Volenec said.

“Growers may notice a slight reduction in forage quality because high-yielding alfalfa is slightly more stemmy, but it’s important to understand that the yield advantages far outweigh this slight reduction in quality,” he said.

“Quality is a low-tier issue with well-managed alfalfa. It’s really about high yield and stand persistence. Quality will follow hand in hand if your fields yield well and have good persistence.”

Replace

At the very least, growers should replace the potassium and phosphorus that is removed from the field during the summer, Volenec said.

“For example, a grower harvests 5 tons of hay from a field this summer, which is 10,000 pounds, with two percent potassium in the tissue,” Volenec said. “This results in the removal of 200 pounds of potassium and, at a minimum, this grower should apply 200 pounds of potassium. Then have the soil tested in the fall and make the necessary adjustments.”

Volenec and Johnson recommend a soil test be taken every three to four years to make sure the plants have adequate nutrients available.

Broadcast application

When applying potassium and phosphorus fertilizer, broadcast application is the route to go, Johnson said.

“Broadcast applications of potassium and phosphorus keep most of the nutrients in the top few inches of soil because these nutrients do not move vertically in most soils,” he said. “Our research shows this is not a concern because most of the fine roots active in nutrient uptake are in the top 2 inches of soil.”

The bottom line is alfalfa yield and stand persistence improve with potassium and phosphorus fertilizer, Volenec said.

“With good balanced nutrition we’ve had stands persist for six to seven years easily with really good yields,” Volenec said.